The present invention relates generally to proton exchange membrane (PEM) electrochemical cell stacks and relates more particularly to a novel PEM electrochemical cell stack.
In certain controlled environments, such as those found in airplanes, submarines and spacecrafts, it is often necessary for oxygen to be furnished in order to provide a habitable environment. An electrolysis cell, which uses electricity to convert water to hydrogen and oxygen, represents one type of device capable of producing quantities of oxygen. One common type of electrolysis cell comprises a proton exchange membrane, an anode positioned along one face of the proton exchange membrane, and a cathode positioned along the other face of the proton exchange membrane. To enhance electrolysis, a catalyst, such as platinum, is typically present both at the interface between the anode and the proton exchange membrane and at the interface between the cathode and the proton exchange membrane. The above-described combination of a proton exchange membrane, an anode, a cathode and associated catalysts is commonly referred to in the art as a membrane electrode assembly.
In use, water is delivered to the anode and an electric potential is applied across the two electrodes, thereby causing the electrolyzed water molecules to be converted into protons, electrons and oxygen atoms. The protons migrate through the proton exchange membrane and are reduced at the cathode to form molecular hydrogen. The oxygen atoms do not traverse the proton exchange membrane and, instead, form molecular oxygen at the anode. (An electrolysis cell, when operated in reverse to generate water and electricity using molecular hydrogen and molecular oxygen as starting materials, is referred to in the art as a fuel cell. Electrolysis cells and fuel cells both constitute electrochemical cells, and all discussion herein pertaining to electrolysis cells is correspondingly applicable to fuel cells.)
Often, a number of electrolysis cells are assembled together in order to meet hydrogen or oxygen production requirements. One common type of assembly is a stack comprising a plurality of stacked electrolysis cells that are electrically connected in series in a bipolar configuration. In a typical stack, each cell includes, in addition to a membrane electrode assembly of the type described above, a pair of multi-layer metal screens, one of said screens being in contact with the outer face of the anode and the other of said screens being in contact with the outer face of the cathode. (In another typical electrolysis cell stack design, the multi-layer metal screen on the anode side is omitted, and the separator is provided with a set of molded or machined grooves for defining a fluid cavity.) The screens are used to form the fluid cavities within a cell for the water, hydrogen and oxygen.
Each cell additionally includes a pair of cell frames, each cell frame peripherally surrounding a screen. The frames are used to peripherally contain the fluids and to conduct the fluids into and out of the screen cavities. Each cell further includes a pair of metal foil separators, one of said separators being positioned against the outer face of the anode screen and the other of said separators being positioned against the outer face of the cathode screen. The separators serve to axially contain the fluids on the active areas of the cell assembly. In addition, the separators and screens together serve to conduct electricity from the anode of one cell to the cathode of its adjacent cell. Plastic gaskets seal the outer faces of the cell frames to the metal separators, the inner faces of the cell frames being sealed to the proton exchange membrane.
The cells of the stack are typically compressed between a spring-loaded rigid top end plate and a bottom base plate. In order to ensure optimal conversion of water to hydrogen and oxygen by each electrolysis cell in a stack, there must be uniform current distribution across the active areas of the electrodes of each cell. Such uniform current distribution requires uniform contact pressure over the active areas of the electrodes. Accordingly, one way in which uniform contact pressure over the entire active areas of the electrodes has been maintained has been to provide an electrically-conductive compression pad between adjacent cells in a stack.
Additional information relating to electrolysis cell stacks includes the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 6,057,053, inventor Gibb, issued May 2, 2000; U.S. Pat. No. 5,466,354, inventors Leonida et al., issued Nov. 14, 1995; U.S. Pat. No. 5,366,823, inventors Leonida et al., issued Nov. 22, 1994; U.S. Pat. No. 5,350,496, inventors Smith et al., issued Sep. 27, 1994; U.S. Pat. No. 5,324,565, inventors Leonida et al., issued Jun. 28, 1994; U.S. Pat. No. 5,316,644, inventors Titterington et al., issued May 31, 1994; U.S. Pat. No. 5,009,968, inventors Guthrie et al., issued Apr. 23, 1991; and Coker et al., “Industrial and Government Applications of SPE Fuel Cell and Electrolyzers,” presented at The Case Western Symposium on “Membranes and Ionic and Electronic Conducting Polymer,” May 17-19, 1982 (Cleveland, Ohio).
Although electrolysis cell stacks of the type described above have proven to be generally satisfactory for their intended purpose, there still remains room for improvement. One such area in which room for improvement exists is with respect to the above-described cell frames. Such cell frames are typically made of an unfilled plastic, such as polysulfone. Polysulfone cell frames are desirable in that they have good electrical insulating properties, they are chemically inert and they can easily be formed by conventional machining methods or by economical molding or thermo-forming techniques. Unfortunately, however, cell frames made of unfilled plastic materials may be fluid incapable of withstanding the considerable structural stresses imposed by high internal pressures required for some electrochemical cell applications. In addition, high internal fluid pressure can cause excessive expansion of the inside diameter of the frame, which can result in the formation of an alternative fluid path preventing proper water distribution in an electrolysis cell.
One approach to this problem has been to add fillers (typically in the form of a reinforcing cloth or reinforcing fibers) to the plastic material to increase the strength of the cell frame. This approach, however, has the drawback that the fillers often add porosity to the cell frame, allowing cell fluids to wick through the frame via the fillers. Another drawback is that the cell fluids have a tendency to react adversely with the filler materials when they come into contact therewith.
Another approach to this problem has been to enclose the cell frames within a fluid containment pressure vessel and to supply a back pressure against the frame stack using an inert fluid so that the differential pressure between the operating pressure inside the cell frames and the external back pressure outside the cell frames is kept to a minimal level. Unfortunately, however, this approach is complicated, expensive and adds a lot of weight to the cell stack, as well as to systems incorporating such cell stacks.
Still another approach to this problem has been to surround the cell frames with reinforcing rings, with each reinforcing ring surrounding a single cell frame and being spaced apart from its adjacent reinforcing rings. Each of said reinforcing rings is typically made of a metal or a rigid plastic and is fitted to the exterior of the cell frame. One drawback with this approach is that, where the reinforcement rings are made of metal, insulating material must be positioned between adjacent rings so that they do not electrically short across each other. Another drawback is that each ring must be individually secured to its corresponding cell frame and move with it when the stack expands or compresses due to temperature and/or creep.
In some instances, when such rings are very thin and narrow, they may become unstable, causing the outside diameter to “oil can” up or down.